Domestication relaxed selective constraints on the yak mitochondrial genome.

Wild yaks roaming in the high-altitude Qinghai-Tibetan plateau have to maintain high metabolic efficiency. However, domestic yaks do not require such high efficiency because of their limited activity. Hence, domestication may have caused the relaxation of selective constraints on the yak mitochondrial genome because mitochondrial mutations are extremely sensitive to energy-related selective pressures. We have tested this hypothesis by analyzing the mitochondrial genomes of 51 domestic yaks and 21 wild yaks. The results show that the ratio of nonsynonymous/synonymous substitutions in mitochondrial protein-coding genes is significantly higher in domestic yak lineages than those of wild yaks. This genetic difference suggests that the relaxation of selective constraints following the domestication in addition to bottlenecks has allowed faster accumulation of nonsilent substitutions in the yak mitochondrial genome, despite its short domestication history.